Tracing: LLM clients + streaming spans on tracing, drop cx threading (#140)

LlmClient methods no longer take an OpenTelemetry Context. llm.request is
now a tracing span (otel.kind=client, llm.provider/operation/model) that
parents on the caller's ambient span; traceparent injection reads the
span's OTel context via OpenTelemetrySpanExt.

The src/ai feature spans (handlers, insight generation, chat, persona
chat, daily summaries, TTS) move to #[instrument] / info_span!, and every
LLM-bearing tokio::spawn carries its parent span with .instrument(): the
streaming chat turn (chat_turn_stream_inner), async chat / persona turns,
background insight jobs and TTS jobs. Sub-spans in the agentic loops
(ai.chat.loop, ai.agentic.loop) instrument each LLM / tool await.

DAO calls keep their explicit Context (derived from the tracing span)
until the DAO migration (#141).

tests/otel_llm_spans.rs pins nesting + traceparent for the blocking and
streaming paths across a spawn on a multi-thread runtime.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/bin/test_daily_summary.rs

@@ -220,9 +220,7 @@ async fn main() -> Result<()> {
 
         println!("Generating summary...");
 
-        let summary = ollama
-            .generate(&opentelemetry::Context::new(), &prompt, Some(system_prompt))
-            .await?;
+        let summary = ollama.generate(&prompt, Some(system_prompt)).await?;
 
         println!("\n📝 GENERATED SUMMARY:");
         println!("─────────────────────────────────────────");
@@ -238,9 +236,7 @@ async fn main() -> Result<()> {
             );
 
             println!("\nGenerating embedding...");
-            let embedding = ollama
-                .generate_embedding(&opentelemetry::Context::new(), &stripped)
-                .await?;
+            let embedding = ollama.generate_embedding(&stripped).await?;
             println!("✓ Embedding generated ({} dimensions)", embedding.len());
 
             println!("Saving to database...");